subrepo: introduce a `repo_rel_or_abs_source` function The `subrepoutil` module has various function to compute the path of a sub-repository compared to the root of the top repository. However, they was no function available to compute the relative path of the repository "source". And we need this information for exchange operation (in our case, `hg outgoing`). The information is currently build using the `repo._subtoppath` hack. We reuse the same logic but in a dedicated function independent of the `repo._subtoppath` hack. Differential Revision: https://phab.mercurial-scm.org/D10434